1. Field of the Invention
This invention relates generally to controlling the output power of a power amplifier. More particularly, the invention relates to a linear power control loop for controlling the output power of an amplifier contained in a portable communication handset. The invention also prevents an over-current condition and detects power amplifier saturation.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset, also referred to as a portable transceiver. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards have evolved, to which these systems must conform. For example, in the United States, many portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme is narrow band offset π/4 differential quadrature phase shift keying (π/4-DQPSK), and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient, non-linear power amplifier can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying amplitude) modulation signal.
Regardless of the type of modulation methodology employed, the output power supplied by the power amplifier must be controlled to provide the most efficient power level for the conditions under which the communication handset is operating. For example, in the GSM communication system, the power amplifier transmits in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. This power control is typically performed using a feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the control input of the power amplifier.
In some other communication systems, the output power is controlled by a signal from the base station with which the portable transceiver is communicating. Typically, in such an arrangement, the base station simply sends a signal to the portable transceiver instructing the portable transceiver to increase or decrease power. In such systems, there is no specific power requirement, just the command to either increase or decrease power output.
Regardless of the type of power control employed, the output of the power amplifier is preferably controlled in precise steps. For communication handsets that use a bipolar transistor power amplifier, the output of the power amplifier is controlled by a control signal that is applied to the base terminal of the final stage (if multiple amplifier stages are employed) of the power amplifier. This is commonly referred to as the “base bias current.”
As the conditions (e.g., temperature, battery voltage, antenna impedance, etc.) under which the communication handset operates vary, the power control loop acts to maintain the output power of the power amplifier constant by adjusting the base bias current. Increasing the base bias current generally causes the output of the power amplifier to increase.
While a conventional power control loop provides some control over the power output, some problems may arise. For example, if the base bias current increases past a certain level, the power amplifier is susceptible to failure. This can happen, for example, if the impedance of the antenna abruptly changes due to, for example, a change in the position of the portable transceiver relative to nearby reflective surfaces.
Another problem with a conventional power control loop is that the ratio of the base bias current to the output power characteristic is non-linear. At higher power levels, the level of the base bias control current must be disproportionately (i.e., non-linearly) raised to achieve a commensurate increase (in dB) in output power. This causes the “loop gain” of the power control loop to decrease at higher output power levels, which lengthens the response time of the power control loop. This manifests as an inability to quickly shut off the transmitter, which is a problem in systems such as GSM in which a burst transmission methodology demands fast power ramp-up and ramp-down times.
Linearizing the power control loop has been previously attempted by inserting a “shaper” stage into the power control loop. The shaper is a filter that is designed to exhibit a non-linear gain that has the inverse characteristic of the ratio of the base bias current to the output power. Unfortunately, it is difficult to obtain an exactly inverse characteristic, and the loop gain of the power control loop still decreases at higher output power, causing the response time of the power control loop to decrease.
Previous power control systems have protected the power amplifier by using various types of power amplifier saturation detection methodologies. Conventional saturation detectors sense the decrease in loop gain by observing a large error signal in the power control loop. Unfortunately, these systems require additional circuitry to reduce the base bias current when saturation is detected. For example, a logical “saturation detect” signal may be directed to a microprocessor or digital signal processor (DSP) in the communication handset. The microprocessor or DSP then directs the power control loop to reduce its target value until the saturation detect signal is cleared. A disadvantage of this type of system is that a single threshold is chosen to determine when the power amplifier has become, or is becoming, saturated. This is problematic because the power amplifier will not operate above this level, while the true saturation point of the power amplifier may be dependent on temperature and other variables that change over time.
Another conventional power control system measures only the base bias current. This protects the power amplifier from burning out, but measuring the base bias current provides only an indirect indication of the output power of the power amplifier.
Therefore it would be desirable to provide a power control loop for a power amplifier that exhibits linear response and that includes saturation detection and over-current protection.